The present invention relates to an image recording apparatus such as a printer, copying apparatus and a composite image recording apparatus. More particularly, the present invention relates/to an image recording apparatus provided with toner image forming means for forming a toner image with a dry developer and provided with an image fixing means for fixing the toner image, the toner, that is, the developer including a thermoplastic resin, carbon pigment or dye or the like.
Conventionally, for the purpose of fixing the toner image, a heat fixing means is used, in which a large quantity of heat is applied to the toner and the sheet bearing the toner image. This heat vaporizes the moisture contained in the sheet. Since the heat quantity applied to the front side of the sheet is not equal to that applied to the backside thereof, the amounts of the evaporation are not equal. This results in the sheet curling. When a sufficient quantity of heat is applied to each side of the sheet, the rigidity of the sheet is decreased with the evaporation of the moisture contained in the sheet. Therefore, the configuration of the curling is determined by the conditions under which the sheet is placed immediately after the sheet is discharged from the heat fixing device.
The occurrence of curling is cumbersome, particularly when the image recording apparatus is equipped with a sorter for automatically sorting the discharged sheets or when the image recording apparatus is of a duplex type wherein images are recorded on both sides of a sheet. This is because there may be a trouble in the transportation of the sheet. In the duplex type image recording apparatus, addition to the unsatisfactory transportation of the sheet to an intermediate tray, a trouble can occur when the sheet is separated from a photosensitive member after an image is formed on the photosensitive member and then transferred to the second side of the sheet. The trouble in the separation may result in a disturbance to the image.
Laid-Open Japanese Patent Application No. 81270/1982 proposes, in order to solve the problem described above, that the sheet itself is improved from the standpoint of reducing the amount of curling. More particularly, the moisture content of a sheet is maintained 5.5-6.5% so as to maintain to a desired rigidity in the sheet. This, however, limits the sheets which are usable, and can not satisfactorily be used with plain paper. In addition, the sheets are vulnerable to changes in ambient conditions, for example, the sheets absorb moisture under relatively high humidity conditions, resulting in less rigidity. Furthermore, when the amount of evaporation increases, the amount of curling may be increased.
U.S. Pat. No. 4,375,327 proposes a heat fixing device including a heat fixing roller and a pressing roller, wherein a separation pawl is provided so as to contact the pressing roller, and the distance between the separation pawl and the nip between the fixing roller and the pressing roller is reduced.
Laid-Open Japanese Patent Application No. 113637/1976 proposes that the speed of sheet transportation be higher after the sheet is discharged from the nip in the fixing device than during its transportation in the fixing device.
However, those proposals are not able to deal with the curling of a particular portion of the sheet or the entirety thereof. Particularly in the case of the duplex image recording, the trailing end of the sheet in the first side printing becomes the leading edge in the second side printing, and therefore, it is necessary to regulate the curling in the neighborhood of the sheet trailing edge in the first side printing. The above two proposals are not satisfactory to meet this necessity. U.S. Pat. No. 4,375,327 mentioned above is effective to regulate a wave-form curling, and the Japanese Laid-Open Patent Application can regulate curling only when the sheet is between the fixing roller and transporting means located downstream thereof. Thus, neither can deal with curling in the neighborhood of the leading or trailing edge of the sheet.
There are other proposals for the purpose of regulating or eliminating curling of the sheet at the discharge of the fixing roller, but they are not satisfactory in order to meet curling from the leading edge to trailing edge.
Laid-Open Japanese Patent Application No. 143339/1978, noting that the sheet curling is the most particular problem in the duplex printing, proposes that a toner image on the first side is temporarily fixed, and the toner images on the two sides are fixed to a sufficient extent when the second side toner image is fixed. During the fixing operation for the first side, that is, the temporary image fixing, the sheet does not receive a large quantity of heat so that the amount of curling is not large, but correspondingly, the image fixing is not sufficient. Those insufficiently fixed toner images may be scraped off by transporting rollers and so on during the sheet transportation for the second side image formation. As will be understood, the conditions for providing satisfactory fixing and the conditions for reduced curling, are contradictory to each other, and the conditions under which both are satisfactory are so limited that reducing amount of the curling by degrading slightly the fixing performance is practically very difficult.
The inventors found Laid-Open Japanese Patent Application No. 24862/1984, of which neither inventor was aware prior to the achievement of the present invention. The fixing device disclosed therein is provided with a water pan located above the fixing mechanism. The pan is heated by the heat emitted from the fixing mechanism after it is sufficiently heated. Thereafter, the water in the pan is heated thereby and is evaporated. In this system, a state is formed outside the copying mechanisms, wherein the water vapor is dispersed in the ambient atmosphere. Then, the sheet after being subjected to the image fixing operation is introduced in the ambience.
However, it requires a long time from the start of the water pan heating to the water therein becoming evaporated. The time is much longer than the time required from actuation of the main switch to the copyable state being reached. Therefore, the sheet curling could not be prevented during a significant time period from the time the copyable state is reached. Since the amount of the water contained in the water pan is necessarily limited, curl prevention is not performed after the water is consumed. It would be possible to replenish the water into the water pan, but this would require complicated mechanisms for the control because the temperature in and of the copying machine is increased by the continuous long operation. Further, the manner in which the curling which will be described hereinafter is prevented is not known.